MHD Stability and Transport Modeling of Negative Triangularity Plasmas in DIII-D

Recent Negative Triangularity (NT) experiments suggest a new class of reactor scenarios that integrate an L-mode edge free of ELMs with a high-performance fusion core. A NT state without deleterious MHD activity at β_N≈3 has been observed in DIII-D. NT plasmas have long been thought to have lower MHD stability limits than similar positive triangularity (PT) plasmas. Ideal MHD simulations of experimental NT equilibria predicting ideal wall limits near β_N = 3.5 are detailed. NT tearing stability (PEST3 and RDCON) is compared with that of the PT hybrid scenario at similar β_N. The FASTRAN code suite has been used to reproduce MHD and transport consistent kinetic equilibria of DIII-D NT experiments. MHD stability optimization is explored using FASTRAN over scans of current density and pressure profile shapes. DIII-D PCS simulations identify attainable plasma shapes (-0.44 average triangularity, elongation 1.66, 16m³) yielding n=0 growth rates low enough to control in upcoming experiments. These stability and transport results taken together indicate that continuous operation is possible at β_N≈3.